Circuit for driving synchronous rectifier device

ABSTRACT

A driving circuit for driving a synchronous rectifier device. The driving circuit may include a controllable charging circuit and a slope sensing circuit. The slope sensing circuit may sense whether an abrupt rising change in a voltage drop from a sensing terminal to a reference ground terminal of the driving circuit is occurring, and provide a slope sensing signal in response to a rising edge of the abrupt rising change in the voltage drop. The controllable charging circuit may receive the slope sensing signal and provide a charging current to a supply terminal of the driving circuit in response to each rising edge of the abrupt rising change in the voltage drop.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims the benefit of CN application No. 201911247505.7filed on Dec. 9, 2019 and incorporated herein by reference.

TECHNICAL FIELD

This disclosure relates generally to electrical circuit, and moreparticularly but not exclusively relates to circuit for drivingsynchronous rectifier device.

BACKGROUND

Synchronous rectifier devices are widely used in various applications,for instance, configured as switching device at secondary side ofisolated converters e.g. LLC resonant converter and flyback converteretc. During the operation of an isolated converter, each time when aprimary switching device at its primary side is turned on to enable acurrent to flow through the primary winding of the isolated converter,an abrupt increase in a voltage drop on the synchronous rectifier deviceat the secondary side may be induced. For example, if a metal oxidesemiconductor field effect transistor (“MOSFET”) is used as thesynchronous rectifier device, an abrupt increase in a drain to sourcevoltage of the MOSFET may be induced due to turn on of the primaryswitching device at the primary side. For this situation, largeovershoot voltage spike in the voltage drop on the synchronous rectifierdevice may occur and result in severe damage of circuit elements in theisolated converter etc. Using conventional RC snubber circuit to absorbsuch large overshoot voltage spike is neither economic nor efficient.

SUMMARY

In accomplishing the above and other objects, there has been provided,in accordance with an embodiment of the present invention, a drivingcircuit for driving a synchronous rectifier device. The driving circuitmay comprise a controllable charging circuit, coupled between a sensingterminal and a supply terminal of the driving circuit, and configured toprovide a charging current to the supply terminal in response to a slopesensing signal; and a slope sensing circuit, coupled between the sensingterminal and a reference ground terminal of the driving circuit, andconfigured to sense whether an abrupt rising change in a voltage dropfrom the sensing terminal to the reference ground terminal of thedriving circuit is occurring, and further configured to provide theslope sensing signal in response to a rising edge of the abrupt risingchange in the voltage drop.

In accordance with an embodiment, the slope sensing circuit may beconfigured to sense a voltage slew rate of the voltage drop, and tocompare the voltage slew rate of the voltage drop with a predeterminedslew rate threshold to provide the slope sensing signal when the voltageslew rate of the voltage drop is higher than the predetermined slew ratethreshold.

In accordance with an embodiment, the controllable charging circuit maybe configured to provide the charging current during a predeterminedcharging duration in response to a slope sensing signal.

In accordance with an embodiment, the controllable charging circuit maycomprise: a charging current generation circuit, having a first terminalcouple to the sensing terminal of the driving circuit and a secondterminal coupled to the supply terminal of the driving circuit through acontrollable switch; a charging duration timing circuit, configured toreceive the slope sensing signal, and to generate a timing pulse of acharging timing signal in response to each abrupt rising change in thevoltage drop from the sensing terminal to the reference ground terminalof the driving circuit, and wherein a pulse width of the timing pulse ofthe charging timing signal is indicative of a predetermined chargingduration; and a controllable switch, having a control terminalconfigured to receive the charging timing signal, and wherein thecontrollable switch is on within the pulse width of each timing pulse ofthe charging timing signal.

In accordance with an embodiment, the slope sensing circuit maycomprise: a voltage slew rate sensing circuit, coupled to the sensingterminal of the driving circuit and configured to generate a sensingcurrent in response to transient change in the voltage drop from thesensing terminal to the reference ground terminal of the drivingcircuit; a current to voltage conversion circuit, coupled to an outputterminal of the voltage slew rate sensing circuit to receive the sensingcurrent and to convert the sensing current into a sensing voltage; and acomparison circuit, having a first terminal coupled to the outputterminal of the current to voltage conversion circuit to receive thesensing voltage, and a second terminal coupled to receive a referencevoltage, wherein the comparison circuit is configured to compare thesensing voltage with the reference voltage to provide the slope sensingsignal.

In accordance with an embodiment, the voltage slew rate sensing circuitmay comprise a capacitive element.

In accordance with an embodiment, the reference voltage may beindicative of a predetermined slew rate threshold.

In accordance with an embodiment, the sensing current has a currentamplitude that is proportional to the slew rate of the voltage drop fromthe sensing terminal to the reference ground terminal of the drivingcircuit.

In accordance with an embodiment, the sensing voltage is proportional tothe sensing current.

In accordance with an embodiment, when the sensing voltage is higherthan or reaches the reference voltage, the slope sensing signal changesfrom a first logic state to a second logic state, and when the sensingvoltage is lower than the reference voltage, the slope sensing signalchanges from the second logic state to the first logic state, and thecontrollable charging circuit may be configured to provide the chargingcurrent in response to the second logic state of the slope sensingsignal.

In accordance with an embodiment, the sensing voltage higher than orreaching the reference voltage indicates that the rising slew rate inthe voltage drop is higher than a predetermined slew rate threshold, andthat the slope sensing circuit has sensed that the abrupt rising changein the voltage drop is occurring.

In accordance with an embodiment, the current to voltage conversioncircuit may be further configured to filter the sensing voltage tooutput the portion of the sensing voltage having positive phase.

In accordance with an embodiment, the current to voltage conversioncircuit may comprise a resistive element, and wherein the sensingcurrent flowing through the resistive element generates the sensingvoltage.

In accordance with an embodiment, the current to voltage conversioncircuit may further comprise a voltage clamping or rectifying circuitthat is configured to clamp the sensing voltage at positive phase.

In accordance with an embodiment, the sensing terminal and the referenceground terminal of the driving circuit may respectively be coupled to afirst terminal and a second terminal of the synchronous rectifierdevice, and the driving circuit may further comprise a driving terminalthat is coupled to a control terminal of the synchronous rectifierdevice.

In accordance with an embodiment, the supply terminal of the drivingcircuit may be coupled to the reference ground terminal through acapacitive energy storage device.

In accordance with an embodiment, the driving circuit may furthercomprise: a logic control circuit, coupled to the supply terminal andconfigured to provide a control signal for controlling on and offswitching of the synchronous rectifier device.

In accordance with an embodiment, the driving circuit may furthercomprise: a driver, coupled to the supply terminal and configured toenhance a driving capacity of the control signal to provide a drivingsignal to a control terminal of the synchronous rectifier device.

In accordance with an embodiment, the sensing terminal of the drivingcircuit may be coupled to an output terminal of an isolated converter,and the reference ground terminal may be coupled to a secondary windingof a transformer of the isolated converter. Or in accordance with analternative embodiment, the sensing terminal of the driving circuit maybe coupled to the secondary winding of the transformer of the isolatedconverter while the reference ground terminal may be coupled to theoutput terminal of the isolated converter.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description of various embodiments of the presentinvention can best be understood when read in conjunction with thefollowing drawings, in which the features are not necessarily drawn toscale but rather are drawn as to best illustrate the pertinent features.

FIG. 1 illustrates a block diagram of a synchronous rectifying circuit100 in accordance with an embodiment of the present invention.

FIG. 2 illustrates a schematic diagram of a driving circuit 102 inaccordance with an embodiment of the present invention.

FIG. 3 illustrates a schematic diagram of a charging duration timingcircuit 203 in accordance with an embodiment of the present invention.

FIG. 4 illustrates a schematic diagram of a charging duration timingcircuit 203 in accordance with an alternative embodiment of the presentinvention.

FIG. 5 illustrates a schematic diagram of a slope sensing circuit 1024in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION

Various embodiments of the present invention will now be described. Inthe following description, some specific details, such as examplecircuits and example values for these circuit components, are includedto provide a thorough understanding of the embodiments. One skilled inthe relevant art will recognize, however, that the present invention canbe practiced without one or more specific details, or with othermethods, components, materials, etc. In other instances, well-knownstructures, materials, processes or operations are not shown ordescribed in detail to avoid obscuring aspects of the present invention.

Throughout the specification and claims, the term “coupled,” as usedherein, is defined as directly or indirectly connected in an electricalor non-electrical manner. The terms “a,” “an,” and “the” include pluralreference, and the term “in” includes “in” and “on”. The phrase “in oneembodiment,” as used herein does not necessarily refer to the sameembodiment, although it may. The term “or” is an inclusive “or”operator, and is equivalent to the term “and/or” herein, unless thecontext clearly dictates otherwise. The term “based on” is not exclusiveand allows for being based on additional factors not described, unlessthe context clearly dictates otherwise. The term “circuit” means atleast either a single component or a multiplicity of components, eitheractive and/or passive, that are coupled together to provide a desiredfunction. The term “signal” means at least one current, voltage, charge,temperature, data, or other signal. Where either a field effecttransistor (“FET”) or a bipolar junction transistor (“BJT”) may beemployed as an embodiment of a transistor, the scope of the words“gate”, “drain”, and “source” includes “base”, “collector”, and“emitter”, respectively, and vice versa. Those skilled in the art shouldunderstand that the meanings of the terms identified above do notnecessarily limit the terms, but merely provide illustrative examplesfor the terms.

FIG. 1 illustrates a block diagram of a synchronous rectifying circuit100 in accordance with an embodiment of the present invention. Thesynchronous rectifying circuit 100 may comprise a synchronous rectifierdevice 101 and a driving circuit 102. The synchronous rectifier device101 may be configured as a switching device at a secondary side of anisolated converter. For instance, in the example of FIG. 1, thesynchronous rectifier device 101 may be controlled by the drivingcircuit 102. The driving circuit 102 may have a sensing terminal VDcoupled to an output terminal OUT of the isolated converter, a referenceground terminal VSS coupled to a secondary winding T2 of a transformer Tof the isolated converter, and a supply terminal VDD coupled to thereference ground terminal VSS via a capacitive energy storage device103. In practical application, a voltage potential (also labeled withVSS) at the reference ground terminal VSS may be used as a referenceground potential VSS of the driving circuit 102 (including all thesub-circuits and elements that the driving circuit 102 may comprise). Afirst terminal D of the synchronous rectifier device 101 may be coupledto the sensing terminal VD of the driving circuit 102. A second terminalS of the synchronous rectifier device 101 may be coupled to thereference ground terminal VSS of the driving circuit 102. A controlterminal G of the synchronous rectifier device 101 may be coupled to adriving terminal VG of the driving circuit 102. In accordance withalternative embodiments of the present invention, the supply terminalVDD may be coupled to the sensing terminal VD of the driving circuit 102through a controllable charging circuit. The sensing terminal VD of thedriving circuit 102 may be coupled to the first terminal D of thesynchronous rectifier device 101 to sense/monitor a voltage potential(also denoted by VD) at the first terminal D of the synchronousrectifier device 101 and draw power, the voltage potential VD at thefirst terminal D of the synchronous rectifier device 101 taking thevoltage potential VSS at the second terminal S of the synchronousrectifier device 101 as a reference potential. In other exemplaryapplications, the sensing terminal VD of the driving circuit 102 may becoupled to the secondary winding T2 of the transformer T of the isolatedconverter and the reference ground terminal VSS of the driving circuit102 may be coupled to an output terminal OUT of the isolated converter.The synchronous rectifier device 101 may provide a current path from thefirst terminal D to the second terminal S when turned on and may switchthe current path off when turned off.

FIG. 2 illustrates a schematic diagram of the driving circuit 102 inaccordance with an embodiment of the present invention. The drivingcircuit 102 may comprise a controllable charging circuit 1021. Thecontrollable charging circuit 1021 may be coupled between the sensingterminal VD of the driving circuit 102 and the supply terminal VDD ofthe driving circuit 102, and may be configured to provide a chargingcurrent Ich to the supply terminal VDD in response to a slope sensingsignal S_(SLP) to charge the capacitive energy storage device 103 toprovide a supply potential VDD. For instance, in response to the slopesensing signal S_(SLP), a current path may be formed in the chargingcircuit 1021 to provide the charging current Ich. In an embodiment, inresponse to the slope sensing signal S_(SLP), the controllable chargingcircuit 1021 may be configured to provide the charging current Ichwithin a predetermined charging duration t_(ch) to the supply terminalVDD. That is to say, in response to the slope sensing signal S_(SLP),the controllable charging circuit 1021 may provide the charging currentIch during the predetermined charging duration t_(ch) and stop providingthe charging current Ich when the predetermined charging duration t_(ch)elapsed. The supply potential VDD may be used as power supply to thedriving circuit 102 (including all the sub-circuits and elements thatthe driving circuit 102 may comprise). The slope sensing signal S_(SLP)may be indicative of an abrupt rising change in a voltage drop VDS fromthe sensing terminal VD of the driving circuit 102 to the referenceground terminal VSS of the driving circuit 102. The voltage drop VDS mayalso be referred to as a voltage potential difference between a voltagepotential at the sensing terminal VD and the reference ground potentialVSS at the reference ground terminal VSS of the driving circuit 102. Inthe exemplary embodiments shown in FIG. 1 and FIG. 2, since the sensingterminal VD and the reference ground terminal VSS of the driving circuit102 are respectively coupled to the first terminal D and the secondterminal S of the synchronous rectifier device 101, the slope sensingsignal S_(SLP) may be indicative of an abrupt rising change in a voltagedrop (also labeled with VDS) on the synchronous rectifier device 101.The voltage drop VDS on the synchronous rectifier device 101 may referto a voltage potential difference between the voltage potential VD atthe first terminal D and the voltage potential VSS at the secondterminal S of the synchronous rectifier device 101.

In accordance with an embodiment of the present invention, the drivingcircuit 102 may further comprise a slope sensing circuit 1024, coupledbetween the sensing terminal VD and the reference ground terminal VSS ofthe driving circuit 102. The slope sensing circuit 1024 may beconfigured to sense whether the voltage drop VDS from the sensingterminal VD to the reference ground terminal VSS of the driving circuit102 is rising abruptly (or in other words, to sense whether abruptrising change in the voltage drop VDS from the sensing terminal VD tothe reference ground terminal VSS of the driving circuit 102 isoccurring). In practical applications, referring to illustrations inFIG. 1 and FIG. 2, when the sensing terminal VD and the reference groundterminal VSS of the driving circuit 102 are respectively coupled to thefirst terminal D and the second terminal S of the synchronous rectifierdevice 101, the slope sensing circuit 1024 may generally be configuredto sense whether the voltage drop VDS on the synchronous rectifierdevice 101 is rising abruptly (or in other words, to sense whetherabrupt rising change in the voltage drop VDS on the synchronousrectifier device 101 is occurring). For instance, in the isolatedconverter application shown in FIG. 1, the synchronous rectifier device101 is used as a switching device to switch on and off at the secondaryside of the isolated converter. Each time the primary side switchingdevice of the isolated converter is turned on, a current path is formedat the primary side of the isolated converter, and the potentialdifference VDS between the first terminal D and the second terminal S ofthe synchronous rectification device 101 will increase/rise sharply (forexample, within tens of nanoseconds, it will rapidly rise from thereference ground potential to 80V) due to mutual inductance on thesecondary side, that is, an abrupt rising change in the voltage drop VDSis occurring. The slope sensing circuit 1024 may be configured to sensethis abrupt rising change in the potential difference VDS between thefirst terminal D and the second terminal S of the synchronousrectification device 101, and provide the slope sensing signal S_(SLP)in response to the rising edge of the abrupt rising change. The slopesensing signal S_(SLP) may be, for example, a narrow pulse signal. Inthis way, each time the primary side switching device is switched fromoff to on, the slope sensing circuit 1024 may sense the rising edge ofthe abrupt rising change in the potential difference VDS between thefirst terminal D and the second terminal S of the synchronous rectifierdevice 101 coupled to the secondary side and generate a narrow pulsesignal S_(SLP). The controllable charging circuit 1021 may be configureto receive the slope sensing signal S_(SLP) and trigger thepredetermined charging duration t_(ch) in response to each narrow pulseof the slope sensing signal S_(SLP). Therefore, each time an abruptrising change in the voltage drop VDS from the sensing terminal VD tothe reference ground terminal VSS of the driving circuit 102 (i.e. thevoltage drop VDS on the synchronous rectifier device 101) occurs, thecontrollable charging circuit 1021 may connect the sensing terminal VDof the driving circuit 102 to the supply terminal VDD of the drivingcircuit 102 in response to the narrow pulse of the slope sensing signalS_(SLP) to charge the capacitive energy storage device 103 within thepredetermined charging duration t_(ch). In this fashion, each time anabrupt rising change in the voltage drop VDS on the synchronousrectifier device 101 occurs, energy of the overshoot voltage spike inthe voltage drop VDS may be absorbed by the capacitive energy storagedevice 103 and converted to the supply potential VDD for supplying thedriving circuit 102, which can not only effectively reject/reduce theovershoot voltage spike in the voltage drop VDS but also make good useof the energy in the overshoot voltage spike. With the driving circuit102 in accordance with various embodiments of the present invention,additional RC snubber circuit conventionally used for absorbing theenergy in the overshoot voltage spike in the voltage drop VDS may beomitted or at least reduced in size. The driving circuit 102 inaccordance with various embodiments of the present invention when usedfor driving a synchronous rectifier device 101 in an isolated convertercan thus not only reduce power consumption but also save cost andimprove system power efficiency.

In accordance with an embodiment of the present invention, as shown inFIG. 2, the controllable charging circuit 1021 may comprise a chargingcurrent generation circuit 201, a controllable switch 202 and a chargingduration timing circuit 203. The charging current generation circuit 201may have a first terminal couple to the sensing terminal VD of thedriving circuit 102 and a second terminal coupled to the supply terminalVDD of the driving circuit 102 through the controllable switch 202. Thecontrollable switch 202 may have a control terminal coupled to an outputterminal of the charging duration timing circuit 203 and configured toreceive a charging timing signal (also labeled with t_(ch) forsimplicity) indicative of the predetermined charging duration t_(ch).The charging duration timing circuit 203 may be configured to receivethe slope sensing signal S_(SLP), and to generate a timing pulse of thecharging timing signal (t_(ch)) in response to each occurrence of theabrupt rising change in the voltage drop VDS from the sensing terminalVD to the reference ground terminal VSS of the driving circuit 102, apulse width of the timing pulse of the charging timing signal may beindicative of the predetermined charging duration t_(ch). Inapplications where the sensing terminal VD and the reference groundterminal VSS of the driving circuit 102 are respectively coupled to thefirst terminal D and the second terminal S of the synchronous rectifierdevice 101, each time when the synchronous rectifier device 101 isswitched from on to off, the charging current generation circuit 201 maygenerate the charging current Ich based on or by utilizing the voltagedrop VDS from the sensing terminal VD to the reference ground terminalVSS of the driving circuit 102 (i.e. the voltage drop VDS from the firstterminal D to the second terminal S of the synchronous rectifier device101). The charging duration timing circuit 203 may accordingly controlthe controllable switch 202 to turn on (or to close) in response to eachabrupt/sharp rising in the voltage drop VDS from the first terminal D tothe second terminal S of the synchronous rectifier device 101 and keepthe controllable switch 202 on during the predetermined chargingduration t_(ch) until the predetermined charging duration t_(ch)terminates (that is the controllable switch 202 may be controlled toturn off/to open when the predetermined charging duration t_(ch)terminates). Each time when the controllable switch 202 is close duringthe predetermined charging duration t_(ch), a charging current path isformed from the sensing terminal VD to the supply terminal VDD toprovide the charging current Ich to the supply terminal VDD to chargethe capacitive energy storage device 103. One of ordinary skill in theart should understand that the charging current generation circuit 201,the controllable switch 202 and the charging duration timing circuit 203and their configuration and connection in FIG. 2 are only illustrativeand not intended to be limiting. The controllable charging circuit 1021may comprise other circuits/elements, for instance, the charging currentgeneration circuit 201 may be coupled to the sensing terminal VD througha high voltage sustaining circuit/element (e.g. a high voltagetransistor having a breakdown voltage that is higher than a peak voltagevalue of the voltage potential VD at the sensing terminal VD), and inFIG. 2, this kind of coupling is illustrated by a dashed line from thecharging current generation circuit 201 to the sensing terminal VDwithout specifically drawing the high voltage sustaining circuit/elementout to avoid obscuring main disclosure of the present invention. Foranother example, a voltage regulating circuit (e.g. LDO) may further becoupled to the supply terminal VDD to regulate the supply potential VDDto other internal supply potential. These kinds of variations should bewell known to one of ordinary skill in the art and do not go beyond thespirit and scope of the present invention.

In accordance with an embodiment of the present invention, as shown inFIG. 3, the charging duration timing circuit 203 may comprise anedge-triggered timer 301. The edge-triggered timer 301 may be configuredto receive the slope sensing signal S_(SLP) and start timing thepredetermined charging duration t_(ch) in response to each narrow pulseof the slope sensing signal S_(SLP) and to output the charging durationtiming signal t_(ch) when the predetermined charging duration t_(ch) hasbeen timed. The edge-triggered timer 301 may further be configured toreset (clear) the edge-triggered timer 301 to zero by the chargingduration timing signal t_(ch) every time the predetermined chargingduration t_(ch) has been timed. In accordance with an alternativeembodiment, as shown in FIG. 4, the charging duration timing circuit 203may comprise a pulse generation circuit 401, configured to receive theslope sensing signal S_(SLP) and to generate a pulse signal having apulse width equal to the predetermined charging duration t_(ch) inresponse to each narrow pulse of the slope sensing signal S_(SLP). Thepulse signal having the pulse width equal to the predetermined chargingduration t_(ch) may be provided as the charging duration timing signalt_(ch). Those of ordinary skill in the art should understand that thecharging duration timing circuit 203 are not confined to thoseillustrated in FIG. 3 and FIG. 4 and may comprise or adopt otherappropriate timing circuit or pulse generation circuit.

In accordance with an embodiment of the present invention, as shown inFIG. 5, the slope sensing circuit 1024 may comprise a voltage slew rate(i.e. voltage changing rate dv/dt) sensing circuit 501, a current tovoltage conversion circuit 502 and a comparison circuit 503. The voltageslew rate sensing circuit 501 may be coupled to the sensing terminal VDof the driving circuit 102 and may be configured to generate a sensingcurrent Id in response to transient change in the voltage drop VDS fromthe sensing terminal VD to the reference ground terminal VSS of thedriving circuit 102. The voltage slew rate sensing circuit 501 maycomprise, for example in an embodiment, a capacitive element 5011 (e.g.a capacitor). The current to voltage conversion circuit 502 may becoupled to an output terminal of the voltage slew rate sensing circuit501 to receive the sensing current Id and to convert the sensing currentId into a sensing voltage Vd. The comparison circuit 503 may have afirst terminal (e.g. the “+” terminal in FIG. 5) coupled to the outputterminal of the current to voltage conversion circuit 502 to receive thesensing voltage Vd, and a second terminal (e.g. the “−” terminal in FIG.5) coupled to receive a reference voltage Vref. The comparison circuit503 may be configured to compare the sensing voltage Vd with thereference voltage Vref to provide the slope sensing signal S_(SLP). Thereference voltage Vref may be indicative of a predetermined slew ratethreshold. In applications where the sensing terminal VD and thereference ground terminal VSS of the driving circuit 102 arerespectively coupled to the first terminal D and the second terminal Sof the synchronous rectifier device 101, each time when the voltage dropVDS from the first terminal D to the second terminal S of thesynchronous rectifier device 101 rising sharply/abruptly due to turn onof the current path at the primary side of the isolated converter, anabrupt rising edge and a large voltage spike present. FIG. 5 alsoillustrates out the waveforms of the voltage drop VDS and the sensingcurrent Id during the transient change in the voltage drop VDS. As canbe seen, each time the current path at the primary side of the isolatedconverter is on, it causes the voltage drop VDS to rise with asharp/abrupt rising edge 505 following with oscillation spikes/rippleswhich finally subside. The first (also the largest) voltage spike in thevoltage drop VDS is marked out with the dashed frame 506 to help betterunderstand the embodiment. The voltage slew rate sensing circuit 501senses the transient change in the voltage drop VDS and generates thesensing current Id, the sensing current Id may have a current amplitudethat is proportional (e.g. with a proportional coefficient) to the slewrate/changing rate of the voltage drop VDS from the first terminal D tothe second terminal S of the synchronous rectifier device 101, theproportional coefficient may be determined by the capacitance of thecapacitive element 5011 in the voltage slew rate sensing circuit 501.That is to say, the current amplitude of the sensing current Id mayincrease with increase in the slew rate/changing rate of the voltagedrop VDS and may decrease with decrease in the slew rate/changing rateof the voltage drop VDS. Therefore, when the sharp/abrupt rising edge505 in the voltage drop VDS generates, a current pulse 507 generatescorrespondingly in the sensing current Id, amplitude of the currentpulse 507 is proportional to the slew rate/changing rate of the voltagedrop VDS at the sharp/abrupt rising edge 505 of the voltage drop VDS,and pulse width of the current pulse 507 is equal to a time durationt_(rise) that the sharp/abrupt rising edge 505 lasts. The sensingcurrent Id may be converted to the sensing voltage Vd that isproportional to the sensing current Id through the current to voltageconversion circuit 502. For example, in an embodiment, the current tovoltage conversion circuit 502 may comprise a resistive element 5021,wherein the sensing current flowing through the resistive element 5021may generate the sensing voltage Vd. In an embodiment, the current tovoltage conversion circuit 502 may further be configured to filter thesensing voltage Vd to filter out a portion of the sensing voltage Vdhaving negative phase and output the portion of the sensing voltage Vdhaving positive phase. For instance, the current to voltage conversioncircuit 502 may further comprise a voltage clamping or rectifyingcircuit 5022 (e.g. illustrated as a diode in FIG. 5), the voltageclamping or rectifying circuit 5022 may be configured to clamp thesensing voltage Vd at positive phase. The sensing voltage Vd may becompared with the reference voltage Vref (indicative of a predeterminedvoltage slew rate/changing rate) by the comparison circuit 503 to outputthe slope sensing signal S_(SLP). When the sensing voltage Vd is higherthan the reference voltage Vref, the slope sensing signal S_(SLP) maychange from a first logic state (e.g. a logic low) to a second logicstate (e.g. a logic high). Since the sensing voltage Vd actually isindicative of the slew rate/changing rate in the voltage drop VDS, thesensing voltage Vd going higher than the reference voltage Vref (i.e.the slope sensing signal S_(SLP) changing from the first logic state tothe second logic state) actually indicates that the rising slewrate/changing rate in the voltage drop VDS is higher than thepredetermined slew rate threshold, and thus indicating that the slopesensing circuit 1024 has determined/sensed that the abrupt rising changein the voltage drop VDS is occurring (i.e. the slope sensing circuit1024 has sensed the sharp/abrupt rising edge 505). In the exemplaryembodiment, the predetermined slew rate threshold may be appropriatelyset according to practical application requirements so that the slopesensing circuit 1024 can differentiate the sharp/abrupt rising edge 505caused by turning on of the current path at the primary side of theisolated converter from rising change in the voltage drop VDS caused byother circumstances. In an embodiment, the second logic state of theslope sensing signal S_(SLP) may trigger the controllable chargingcircuit 1021 to provide the charging current Ich during thepredetermined charging duration t_(ch) to the supply terminal VDD. Thetime duration t_(rise) that the sharp/abrupt rising edge 505 lasts isquite short, and therefore the slope sensing signal S_(SLP) may have anarrow pulse in response to each sharp/abrupt rising edge 505 of thevoltage drop VDS from the first terminal D to the second terminal S ofthe synchronous rectifier device 101. In this fashion, the slope sensingcircuit 1024 may accurately detect each sharp/abrupt rising edge 505 ofthe voltage drop VDS from the first terminal D to the second terminal Sof the synchronous rectifier device 101 caused by turning on of thecurrent path at the primary side of the isolated converter, and issuethe narrow pulse having the second logic state of the slope sensingsignal S_(SLP) to control the controllable charging circuit 1021 toabsorb the large voltage spike in the voltage drop VDS and convert itinto the supply potential VDD.

In accordance with and embodiment of the present invention, turning backto FIG. 2, the driving circuit 102 may further comprise a logic controlcircuit 1022 and a driver 1023. The logic control circuit 1022 and thedriver 1023 may be coupled to the supply terminal VDD of the drivingcircuit 102 to receive the supply potential VDD. The logic controlcircuit 1022 may be configured to provide a control signal CTL forcontrolling on and off switching of the synchronous rectifier device101. The driver 1023 may be configured to enhance the driving capacityof the control signal CTL to provide a driving signal (also labeled withVG) to the driving terminal VG (or to the control terminal G of thesynchronous rectifier device 101).

The advantages of the various embodiments of the present invention arenot confined to those described above. These and other advantages of thevarious embodiments of the present invention will become more apparentupon reading the whole detailed descriptions and studying the variousfigures of the drawings.

From the foregoing, it will be appreciated that specific embodiments ofthe present invention have been described herein for purposes ofillustration, but that various modifications may be made withoutdeviating from the technology. Many of the elements of one embodimentmay be combined with other embodiments in addition to or in lieu of theelements of the other embodiments. Accordingly, the present invention isnot limited except as by the appended claims.

What is claimed is:
 1. A driving circuit for driving a synchronousrectifier device, comprising: a controllable charging circuit, coupledbetween a sensing terminal and a supply terminal of the driving circuit,and configured to provide a charging current to the supply terminal inresponse to a slope sensing signal; and a slope sensing circuit, coupledbetween the sensing terminal and a reference ground terminal of thedriving circuit, and configured to sense whether an abrupt rising changein a voltage drop from the sensing terminal to the reference groundterminal of the driving circuit is occurring, and further configured toprovide the slope sensing signal in response to a rising edge of theabrupt rising change in the voltage drop.
 2. The driving circuit ofclaim 1, wherein the slope sensing circuit is configured to sense avoltage slew rate of the voltage drop, and to compare the voltage slewrate of the voltage drop with a predetermined slew rate threshold, andto provide the slope sensing signal when the voltage slew rate of thevoltage drop is higher than or reaches the predetermined slew ratethreshold.
 3. The driving circuit of claim 1, wherein the controllablecharging circuit is configured to provide the charging current during apredetermined charging duration in response to the slope sensing signal.4. The driving circuit of claim 1, wherein the controllable chargingcircuit comprises: a charging current generation circuit, having a firstterminal couple to the sensing terminal of the driving circuit and asecond terminal coupled to the supply terminal of the driving circuitthrough a controllable switch; a charging duration timing circuit,configured to receive the slope sensing signal, and to generate a timingpulse of a charging timing signal in response to each abrupt risingchange in the voltage drop from the sensing terminal to the referenceground terminal of the driving circuit, and wherein a pulse width of thetiming pulse of the charging timing signal is indicative of apredetermined charging duration; and the controllable switch, having acontrol terminal configured to receive the charging timing signal, andwherein the controllable switch is on within the pulse width of eachtiming pulse of the charging timing signal.
 5. The driving circuit ofclaim 1, wherein the slope sensing circuit comprises: a voltage slewrate sensing circuit, coupled to the sensing terminal of the drivingcircuit and configured to generate a sensing current in response totransient change in the voltage drop from the sensing terminal to thereference ground terminal of the driving circuit; a current to voltageconversion circuit, coupled to an output terminal of the voltage slewrate sensing circuit to receive the sensing current and to convert thesensing current into a sensing voltage; and a comparison circuit, havinga first terminal coupled to the output terminal of the current tovoltage conversion circuit to receive the sensing voltage, and a secondterminal coupled to receive a reference voltage, wherein the comparisoncircuit is configured to compare the sensing voltage with the referencevoltage to provide the slope sensing signal.
 6. The driving circuit ofclaim 5, wherein the voltage slew rate sensing circuit comprises acapacitive element.
 7. The driving circuit of claim 5, wherein thereference voltage is indicative of a predetermined slew rate threshold.8. The driving circuit of claim 5, wherein the sensing current has acurrent amplitude that is proportional to the slew rate of the voltagedrop from the sensing terminal to the reference ground terminal of thedriving circuit.
 9. The driving circuit of claim 5, wherein the sensingvoltage is proportional to the sensing current.
 10. The driving circuitof claim 5, wherein when the sensing voltage is higher than or reachesthe reference voltage, the slope sensing signal changes from a firstlogic state to a second logic state, and wherein when the sensingvoltage is lower than the reference voltage, the slope sensing signalchanges from the second logic state to the first logic state, andwherein the controllable charging circuit is configured to provide thecharging current in response to the second logic state of the slopesensing signal.
 11. The driving circuit of claim 5, wherein the sensingvoltage higher than or reaching the reference voltage indicates that therising slew rate in the voltage drop is higher than a predetermined slewrate threshold, and that the slope sensing circuit has sensed that theabrupt rising change in the voltage drop is occurring.
 12. The drivingcircuit of claim 5, wherein the current to voltage conversion circuit isfurther configured to filter the sensing voltage to output the portionof the sensing voltage having positive phase.
 13. The driving circuit ofclaim 5, wherein the current to voltage conversion circuit comprises aresistive element, and wherein the sensing current flowing through theresistive element generates the sensing voltage.
 14. The driving circuitof claim 13, wherein the current to voltage conversion circuit furthercomprises a voltage clamping or rectifying circuit that is configured toclamp the sensing voltage at positive phase.
 15. The driving circuit ofclaim 1, wherein the sensing terminal and the reference ground terminalof the driving circuit are respectively coupled to a first terminal anda second terminal of the synchronous rectifier device, and wherein thedriving circuit further comprises a driving terminal that is coupled toa control terminal of the synchronous rectifier device.
 16. The drivingcircuit of claim 1, wherein the supply terminal is coupled to thereference ground terminal through a capacitive energy storage device.17. The driving circuit of claim 1, further comprising: a logic controlcircuit, coupled to the supply terminal and configured to provide acontrol signal for controlling on and off switching of the synchronousrectifier device.
 18. The driving circuit of claim 17, furthercomprising: a driver, coupled to the supply terminal and configured toenhance a driving capacity of the control signal to provide a drivingsignal to a control terminal of the synchronous rectifier device. 19.The driving circuit of claim 1, wherein the sensing terminal is coupledto an output terminal of an isolated converter, and wherein thereference ground terminal is coupled to a secondary winding of atransformer of the isolated converter.
 20. The driving circuit of claim1, wherein the sensing terminal is coupled to a secondary winding of atransformer of an isolated converter, and wherein the reference groundterminal is coupled to an output terminal of the isolated converter.